Carbon based nanomaterials, namely carbon nanotubes and graphene, exhibit the highest charge carrier mobilities of any semiconductor, and are structurally and chemically extremely robust. Despite these advantageous properties however, there are no known examples where carbon materials genuinely out-perform conventional semiconductors in their own right when incorporated within an optimized optoelectronic device. Perovskite based solar cells have recently emerged at the forefront of photovoltaics research, but currently employ a low mobility amorphous hole transporter, which in part limits the efficiency through a high series resistance. Here, we report a new approach for achieving efficient hole extraction in perovskite solar cells by combining polymer-functionalized single-walled carbon nanotubes (SWNTs) within the amorphous organic hole-conductor matrix.
By means of polymer (P3HT) functionalization we are able to solution-process the generally insoluble SWNTs, and simultaneously tune their electronic properties such that they become highly selective for p-type charges (holes). We observe that the random dispersion of SWNTs in the dedicated hole transport material enhances the performance of perovskite solar cells. Interestingly, we find that a two-layer deposition of SWNTs and hole transport material leads to an even larger improvement yielding an efficiency of 12.8%. This seems to indicate that highly efficient hole transfer occurs from the perovskite directly to the nanotubes when the SWNTs directly contact the perovskite interface. We confirm this finding by photoinduced absorption studies, which show a distinct bleaching of the ground state absorption of the SWNTs. We infer from these results that the role of the SWNTs goes beyond increasing the conductivity of the hole transporting layer, instead we come to believe that most of the hole transfer occurs rapidly through the SWNT network. To test this hypothesis, we replace the dedicated hole transporting material (spiro-OMeTAD) with an electronically inert polymer (PMMA), and find that carbon nanotubes alone are able to selectively and efficiently transfer holes in perovskite solar cells achieving a power-conversion efficiency of up to 14.2%.With this we have demonstrated a means by which carbon nanotubes can be employed as a highly effective p-type charge collection layer in thin film solar cells. This concept may soon prove to be effective for a wide range of photovoltaic systems. In particular the inherent chemical and thermal robustness of SWNTs may be a key aspect for achieving long-term stability of performance and device integrity of hybrid solar cells. Beyond broad usefulness in photovoltaics, this concept may be just as relevant for charge injection layers in light emitting diodes or organic transistors. This work will certainly encourage renewed effort in achieving application specific properties from SWNTs through electronic modification via surfactant nanotube interactions.